1. Field of the Invention
The present invention relates generally to apparatus for cooling heat generating electronic components, and, in a preferred embodiment thereof, more particularly relates to a heat pipe-based molded heat exchanger structure for use in conjunction with a heat generating electronic component, such as a processor, in a computer.
2. Description of Related Art
As small portable computers, such as the increasingly popular notebook computer, have become faster and more powerful their internally generated operating heat needing to be dissipated has correspondingly increased to the extent that adequate heat dissipation has become a major design concern in these more powerful compact computing devices.
The primary heat generating components in notebook computers, such as the processor, power supply, hard drive and the like, are typically disposed in the base housing portion of the computer to which the display screen-carrying lid housing portion is pivotally secured for movement relative thereto between open and closed positions. With the continuing trend toward higher power densities in electronic equipment, particularly in the case of microprocessor devices such as computers, temperature control becomes critical to successful system design.
Portable notebook computers, due to their combination of high operating heat generation and low volume, have proven to present particularly difficult design problems from an overall heat dissipation standpoint. Higher capacity batteries, higher bus speeds, integrated AC adapters, and greater expandability have combined to increase heat generation in notebook computers while at the same time their volume continues to shrink.
For electronic devices such as computers to operate efficiently they must be kept below certain specified operating temperatures. In the case of low powered devices, or in less temperature-sensitive devices, this may require little design effort. However, for higher powered electronic components it is sometimes necessary to provide a direct path for heat removal. This task becomes an integral part of the design process in working with the device. In these situations it is necessary to provide a thermal path from the interior of the device to the outside environment. The cooling path must have a low thermal resistance in order to efficiently transport heat from the source to the outside environment. The thermal resistance of the path is determined by the thermal conductivity of each element in the path and by the effective thermal conductivity of the interfaces between the elements.
Personal computers frequently employ two types of thermal paths. Most non-portable systems, and some notebook computers, use a forced convection path in which a fan forces moving air over components. Some notebook computers use a combination of natural heat convection and conduction. This is advantageous for notebook computers, as it eliminates the need for a fan. Fans cause problems in notebook computers due to cost, size limitations, reliability concerns, battery power consumption and audible noise.
On the other hand, the elimination of the fan also poses a problem. Without moving air removing heat from the system, it is often necessary to provide a continuous conductive path to the outside of the notebook computer. This poses a further problem since when the heat reaches the outside of the system it can come into contact with the user. If the heat is sufficiently concentrated this can cause uncomfortably hot spots on the surface of the notebook computer. There are human factors specifications which provide temperature limits for areas that the user may be able to contact, and these specification limits must not be exceeded.
In one previously proposed design incorporating a combination of heat conduction and natural conduction to dissipate heat from a processor board in a portable notebook computer, a cooling path from the processor board to the exterior of the computer housing was formed using a thermosyphoning heat pipe in conjunction with a cast magnesium secondary heat exchanger. The underside of the magnesium heat exchanger has a cast-in trough which receives the heat pipe which is held in place within the trough by a thermally conductive epoxy adhesive material which contacts roughly half of the perimeter of the heat pipe.
A first portion of the cast magnesium heat exchanger is suitably held against the heat generating processor board within the base housing of the computer, while a finned second portion of the heat exchanger extends into a slotted, outwardly projecting exterior touch guard portion assembled to the balance of the plastic computer housing, with an air gap being formed between the slotted touch guard structure and the surface of the finned heat exchanger portion. During operation of the computer, heat generated by the processor board is transferred to one end of the heat pipe via the first portion of the magnesium heat exchanger and transferred therefrom through the heat pipe to the finned heat dissipation portion of the heat exchanger for transfer therefrom to ambient air external to the computer housing.
While this previously proposed design adequately transfers processor heat from the interior of the computer to the outside of the computer, it has several disadvantages from both heat transfer and assembly standpoints. For example, the transfer of heat from the heat pipe to the outside air is impaired by several factors. Specifically, (1) only approximately half of the heat pipe perimeter is in contact with the epoxy material used to thermally communicate the heat pipe with the magnesium heat exchanger, (2) the epoxy material has a relatively low thermal conductivity compared to the heat pipe and the heat exchanger, and (3) the air gap between the magnesium heat exchanger and the slotted exterior housing portion undesirably acts as a thermal insulator impeding release of processor heat to the exterior of the computer.
Additionally, the fabrication and assembly process for the heat pipe/heat exchanger structure is a rather involved, labor intensive one which includes (1) casting the magnesium heat exchanger, (2) drilling and tapping the heat exchanger for mounting on the processor board, (3) bending the heat pipe to fit the shape of the trough in the underside of the heat exchanger, (4) epoxying the bent heat pipe into the trough by hand and allowing the epoxy to cure, (5) molding the slotted "touch guard" portion of the computer housing as part of the exterior plastics, (6) assembling the heat exchanger to the processor board, and (7) assembling the touch guard to the heat exchanger.
In addition to the above mentioned heat transfer impairments and fabricational complexity of this previously proposed heat pipe-based heat exchanger structure the use of a magnesium casting to form the heat sink portion of the structure poses a corrosion risk due to its assembly to other galvanicly active materials. It can thus be seen that, in a heat pipe-based heat exchanger of the general type described above, it would be desirable to eliminate or at least substantially reduce these problems, limitations and disadvantages.